Now its time to PRESS that seam.. so here is my advice..
You are pressing, not ironing.. keeping that in mind, here are the steps..
In the following pics, I want to press towards the green or darker piece,
I lay the piece on my ironing board with the green piece on top
First put your iron straight down on the seam line and press it.
Then, using your iron slightly above the fabric, open the seam, pushing the top piece away from you. As the pieces open up, lightly lay the iron on it and press it open..
Do not lay your iron on the first piece and push against the seam to press it open, this will distort the seam line.